Precipitation hardened fe-ni alloy

ABSTRACT

Provided is a precipitation hardened Fe—Ni alloy having the following constitutions: (1) the precipitation hardened Fe—Ni alloy including: from 0.01 to 0.08% by mass of C, from 0.02 to 1.0% by mass of Si, not more than 1.0% by mass of Mn, from 36.0 to 41.0% by mass of Ni, 14.0 or more and less than 20.0% by mass of Cr, from 0.01 to 3.0% by mass of Mo, from 0.1 to 1.0% by mass of Al, from 1.0 to 2.5% by mass of Ti, and from 2.0 to 3.5% by mass of Nb, with the balance being Fe and unavoidable impurities; (2) the precipitation hardened Fe—Ni alloy satisfying the following formulae: Ni≧6×Nb+17 and Nb/(Ti+Al)≧0.8.

FIELD OF THE INVENTION

The present invention relates to a precipitation hardened Fe—Ni alloy.More specifically, the invention relates to a precipitation hardenedFe—Ni alloy having high strength and excellent corrosion resistance.

BACKGROUND OF THE INVENTION

A precipitation hardened stainless steel is a steel in which elementssuch as Cu, Al, Ti, Nb, and Mo are added to achieve precipitationhardening and has both of high corrosion resistance and high strength.Particularly, an austenite precipitation hardened stainless steelrepresented by A286 alloy (SUH660) is an alloy excellent in both ofcorrosion resistance and strength among Fe based alloys. However, forusing the austenite precipitation hardened stainless steel as a memberrequiring high strength in marine environment, it is insufficient inboth of corrosion resistance and strength.

On the other hand, in Fe—Ni alloys, alloys to which Ti, Al, and Nb areadded have been hitherto proposed.

For example, Patent Document 1 (Example 1) discloses a nickel-iron basedalloy comprising, in terms of % by weight, 0.027% of C, 0.08% of Mn,0.10% of Si, 0.001% of P, 0.005% of S, 15.81% of Cr, 39.89% of Ni, 2.83%of Nb, 1.61% of Ti, 0.3% of Al, and 0.0041% of B, with the balance beingFe and unavoidable impurities.

Patent Document 2 (No. 1) discloses an Ni based alloy comprising, interms of % by weight, 0.017% of C, 0.15% of Si, 0.14% of Mn, 0.010% ofP, 0.003% of S, 40.32% of Ni, 16.20% of Cr, 1.02% of Mo, 0.25% of Al,0.95% of Ti, and 2.71% of Nb, with the balance being Fe and unavoidableimpurities.

The document describes a fact that the alloy has high strength from roomtemperature till extremely low temperature and can suppress HAZ crackingby such a composition.

Furthermore, Patent Document 3 (alloy #7) discloses a high-strengthcorrosion-resistant alloy comprising, in terms of % by weight, 44.2% ofNi, 19.5% of Cr, 3.4% of Mo, 2.0% of Cu, 0.006% of C, 0.3% of Al, 3.8%of Nb, and 1.6% of Ti, with the balance being Fe.

The document describes a fact that high strength is obtained byprecipitating predetermined amounts of the γ′ phase and the γ″ phase byannealing and aging treatments.

In Patent Document 1, Mo and Cu are not added and corrosion resistanceis insufficient. In Patent Document 2, strength is insufficient owing tothe balance among Ni, Nb, Ti, and Al. In Patent Document 3, strength ofNi and Nb are high and the raw material costs and the production coststhereof are high.

[Patent Document 1] JP-A-47-042414

[Patent Document 2] JP-A-03-097823

[Patent Document 3] JP-T-2009-515053 (the term “JP-T” as used hereinmeans a published Japanese translation of a PCT patent application)

SUMMARY OF THE INVENTION

A problem to be solved by the present invention is to provide aprecipitation hardened Fe—Ni alloy having both of high corrosionresistance and high hardness.

In order to solve the above problem, the gist of the invention is thatthe precipitation hardened Fe—Ni alloy according to the invention hasthe following constitutions.

(1) the precipitation hardened Fe—Ni alloy comprising:

from 0.01 to 0.08% by mass of C,

from 0.02 to 1.0% by mass of Si,

not more than 1.0% by mass of Mn,

from 36.0 to 41.0% by mass of Ni,

14.0 or more and less than 20.0% by mass of Cr,

from 0.01 to 3.0% by mass of Mo,

from 0.1 to 1.0% by mass of Al,

from 1.0 to 2.5% by mass of Ti, and

from 2.0 to 3.5% by mass of Nb,

with the balance being Fe and unavoidable impurities;

(2) the precipitation hardened Fe—Ni alloy satisfying the followingformulae (1) and (2):

Ni≧6×Nb+17   (1)

Nb/(Ti+Al)≧0.8   (2).

It is preferred that the precipitation hardened Fe—Ni alloy satisfiesthe following formula (3):

Cr+3Mo+5Cu≧19   (3).

When predetermined amounts of Nb, Al, and Ti are added to aprecipitation hardened Fe—Ni alloy, the γ′ phase (Ni₃(Al, Ti, Nb)) andthe γ″ phase (Ni₃Nb) containing Nb as a constituent element areprecipitated by a solution heat treatment and an aging treatment.

At this time, when the Nb content is optimized so as to satisfy theformula (2), the precipitation amount of the γ″ phase is increased.Therefore, high strength can be obtained as compared with theconventional alloys.

On the other hand, as the addition amount of Nb increases, the Lavesphase (Fe₂Nb) is prone to remain after the solution heat treatment. Whenthe Laves phase remains in a large amount, the Nb amount necessary forprecipitation hardening in the matrix decreases. As a result, necessaryhardness cannot be obtained even when the aging treatment is performed.

Contrarily, when the Ni content is optimized so as to satisfy theformula (1), the remaining of the Laves phase can be suppressed afterthe solution heat treatment.

Furthermore, when predetermined amounts of Cr and Mo are added to theprecipitation hardened Fe—Ni alloy or predetermined amounts of Cr, Moand Cu are added to the precipitation hardened Fe—Ni alloy, highcorrosion resistance is obtained with maintaining high strength.Particularly, when the contents of Cr, Mo and Cu are optimized so as tosatisfy the formula (3), high corrosion resistance is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows optical microscopic pictures of the materials after asolution heat treatment obtained in Example 5 and Comparative Example 4.

DETAILED DESCRIPTION OF THE INVENTION

The following will describe one embodiment of the invention in detail.

[1. Precipitation Hardened Fe—Ni Alloy] [1.1. Main Constituent Elements]

The precipitation hardened Fe—Ni alloy according to the inventioncontains the following elements, with the balance being Fe andunavoidable impurities. Kinds of the addition elements, compositionranges thereof, and reasons for the limitation thereof are as follows.

(1) C: From 0.01 to 0.08% by Mass

C is an element effective for forming a carbide together with Nb and Tito enhance the strength. Moreover, it suppresses crystal graincoarsening at a solution heat treatment. For obtaining such effects, theC content is necessarily 0.01% by mass or more. The C content is furtherpreferably 0.04% by mass or more.

On the other hand, when the C content becomes excessive, toughness andductility are lowered. Moreover, when a large amount of the carbide isformed, corrosion resistance is remarkably lowered. For suppressing thelowering of the toughness and ductility and the corrosion resistance,the C content is necessarily 0.08% by mass or less. The C content ispreferably 0.07% by mass or less.

(2) Si: From 0.02 to 1.0% by Mass

Si is effective as a deoxidizing element at the time of ingoting. Forobtaining such an effect, the Si content is necessarily 0.02% by mass ormore.

On the other hand, when the Si content becomes excessive, the toughnessis lowered. Therefore, the Si content is necessarily 1.0% by mass orless.

(3) Mn: Not More than 1.0% by Mass

Similarly to Si, Mn is effective as a deoxidizing element at the time ofingoting. However, when a large amount thereof is added, oxidationresistance at a high temperature is lowered. Moreover, excessive Mn alsolowers corrosion resistance. Therefore, the Mn content is necessarily1.0% by mass or less.

(4) Ni: From 36.0 to 41.0% by Mass

Ni is essential as an austenite-forming element. Also, Ni makes thealloy age-hardened through precipitation of the γ′ phase (Ni₃(Al, Ti,Nb)) and the γ″ phase (Ni₃Nb) together with Ti, Al, and Nb by the agingtreatment. For obtaining such an effect, the Ni content is necessarily36.0% by mass or more. The Ni content is more preferably 37.0% by massor more.

On the other hand, when the Ni content becomes excessive, the rawmaterial costs are increased. Therefore, the Ni content is necessarily41.0% by mass or less. The Ni content is more preferably 40.0% by massor less, further preferably 39.0% by mass or less.

(5) Cr: 14.0 or More and Less than 20.0% by Mass

Cr is an inevitable component for enhancing the corrosion resistance ofthe precipitation hardened Fe—Ni alloy. For obtaining such an effect,the Cr content is necessarily 14.0% by mass or more.

However, Cr is a ferrite-forming element and, when the Cr contentbecomes excessive, structural stability is lowered. Also, excessive Crlowers hot workability. Therefore, the Cr content is necessarily lessthan 20.0% by mass. The Cr content is more preferably 18.0% by mass orless, further preferably 17.0% by mass or less.

(6) Mo: From 0.01 to 3.0% by Mass

Mo improves the corrosion resistance (particularly pitting resistance)through solution into the parent phase. For obtaining such an effect,the Mo content is necessarily 0.01% by mass or more.

On the other hand, when the Mo content becomes excessive, the Lavesphase (Fe₂(Mo, Nb)) is precipitated at the time of the aging treatmentand the precipitation amounts of the γ′ phase and the γ″ phase aredecreased. As a result, the strength of the alloy is lowered. Therefore,the Mo content is necessarily 3.0% by mass or less. The Mo content ispreferably 2.0% by mass or less.

(7) Al: From 0.1 to 1.0% by Mass

Al makes the alloy age-hardened through precipitation of the γ′ phase(Ni₃(Al, Ti, Nb)) together with Ni, Ti, and Nb. For obtaining such aneffect, the Al content is necessarily 0.1% by mass or more.

When the Al content becomes excessive, the hot workability is lowered.Therefore, the Al content is necessarily 1.0% by mass or less. The Alcontent is preferably 0.5% by mass or less.

(8) Ti: From 1.0 to 2.5% by Mass

Ti makes the alloy age-hardened through precipitation of the γ′ phase(Ni₃(Al, Ti, Nb)) together with Ni, Al, and Nb. For obtaining such aneffect, the Ti content is necessarily 1.0% by mass or more. The Ticontent is preferably 1.5% by mass or more, more preferably 1.8% by massor more.

On the other hand, when the Ti content becomes excessive, the hotworkability is lowered. Therefore, the Ti content is necessarily 2.5% bymass or less.

(9) Nb: From 2.0 to 3.5% by Mass

Nb makes the alloy age-hardened through precipitation of the γ′ phase(Ni₃(Al, Ti, Nb)) and the γ″ phase (Ni₃Nb) together with Ni. Forobtaining such an effect, the Nb content is necessarily 2.0% by mass ormore.

On the other hand, when the Nb content becomes excessive, a coarse Lavesphase remains after the solution heat treatment and the precipitationamounts of the γ′ phase and the γ″ phase are decreased. As a result,required strength and hardness are not obtained. Therefore, the Nbcontent is necessarily 3.5% by mass or less. The Nb content is morepreferably 3.0% by mass or less.

[1.2. Auxiliary Constituent Elements]

The precipitation hardened Fe—Ni alloy according to the invention mayfurther contain one kind or two or more kinds of the following auxiliaryconstituent elements in addition to the aforementioned main constituentelements. Kinds of the addition elements, composition ranges thereof,and reasons for the limitation thereof are as follows.

(10) B: From 0.0005 to 0.01% by Mass

B has an effect of enhancing the hot workability by adding B in a smallamount. Also, the precipitation of the η phase at a grain boundary canbe suppressed by the presence of B at the grain boundary. For obtainingsuch an effect, the B content is preferably 0.0005% by mass or more. TheB content is further preferably 0.0010% by mass or more. The B contentis particularly preferably 0.0020% by mass or more.

On the other hand, when the Ni content becomes excessive, the hotworkability is lowered. Therefore, the B content is preferably 0.01% bymass or less. The B content is further preferably 0.008% by mass orless.

(11) Cu: From 0.05 to 1.0% by Mass

Cu has an effect of enhancing the corrosion resistance in anon-oxidative corrosive environment. For obtaining such an effect, theCu content is preferably 0.05% by mass or more. The Cu content isfurther preferably 0.10% by mass or more.

On the other hand, when the Cu content becomes excessive, the hotworkability is lowered. Therefore, the Cu content is preferably 1.0% bymass or less.

(12) V: From 0.05 to 1.0% by Mass

As is the case with Nb and Ti, V forms a carbide to enhance thestrength. Also, the precipitation amounts of the γ′ phase and the γ″phase are increased through reducing the ratio of Nb in the carbide. Forobtaining such effects, the V content is preferably 0.05% by mass ormore.

On the other hand, when the V content becomes excessive, the toughnessand the processability are lowered. Therefore, the V content ispreferably 1.0% by mass or less.

(13) Zr, Ta, W, Hf, Mg, and/or REM: From 0.001 to 0.50% by Mass

Zr, Ta, W, Hf, Mg, and REM (Rare Earth Metal) have an effect onmicronization of the carbide and micronization of crystal grains. Forobtaining such an effect, a total content of these elements ispreferably 0.001% by mass or more.

On the other hand, when the content of these elements becomes excessive,the toughness is lowered. Therefore, the total content of these elementsis preferably 0.50% by mass or less.

Incidentally, any one of these elements may be added or two or morethereof may be used in combination.

(14) Ca: From 0.0005 to 0.01% by Mass

Ca improves machinability. For obtaining such an effect, the Ca contentis preferably 0.0005% by mass or more.

On the other hand, when the Ca content becomes excessive, the hotworkability is lowered. Therefore, the Ca content is preferably 0.01% bymass or less.

[1.3. Component Balance]

The precipitation hardened stainless steel according to the inventionnecessarily satisfies the following formulae (1) and (2), in addition tothe requirement that the constituent elements are present in theaforementioned ranges.

Moreover, for obtaining high corrosion resistance, the precipitationhardened stainless steel preferably further satisfies the followingformula (3).

Ni≧6×Nb+17   (1)

Nb/(Ti+Al)≧0.8   (2)

Cr+3Mo+5Cu≧19   (3)

[1.3.1. Formula (1)]

The formula (1) is relevant to the amount of the Laves phase after thesolution heat treatment. When the Ni amount and the Nb amount areoptimized so as to satisfy the formula (1), the Laves phase (Fe₂Nb) canbe completely dissolved after the solution heat treatment. As a result,the precipitation amounts of the γ′ phase and the γ″ phase at the timeof the aging treatment are increased and thereby the strength of thealloy is enhanced.

The formula (1) is more preferably Ni≧6×Nb+18.0, further preferablyNi≧6×Nb+20.0.

[1.3.2. Formula (2)]

The formula (2) is relevant to the amount of the γ″ phase at the time ofthe aging treatment. When the amounts of Nb, Ti, and Al are optimized soas to satisfy the formula (2), the precipitation amount of the γ″ phaseis increased and thereby further enhancing the strength of the alloy.

[1.3.3. Formula (3)]

The formula (3) is relevant to the corrosion resistance of theprecipitation hardened Fe—Ni alloy. Cr, Mo, and Cu all have an effect ofenhancing the corrosion resistance of the precipitation hardened Fe—Nialloy. Particularly, when the contents of these elements are optimizedso as to satisfy the formula (3), high corrosion resistance is exhibitedwith maintaining high strength.

[1.4. 0.2% Offset Yield Strength]

When individual components are optimized as mentioned above and asuitable solution heat treatment is performed, the Laves phase is almostcompletely dissolved in the matrix. When such a material is subjected toa suitable aging treatment, large amounts of the γ′ phase and the γ″phase are precipitated. As a result, the 0.2% offset yield strength atroom temperature becomes 850 MPa or more. When the components and theheat treatment conditions are further optimized, the 0.2% offset yieldstrength at room temperature becomes 900 MPa or more or 950 MPa or more.

[1.5. Area Percentage of Carbide]

The precipitation hardened Fe—Ni alloy according to the invention ispreferably one in which an area percentage of the carbide after thesolution heat treatment is 0.4% or more. At the time of the solutionheat treatment, the coarsening of crystal grains can be suppressed whena predetermined amount of the carbide is dispersed in the matrix.

Here, the “area percentage of the carbide” means a ratio of area of thecarbide to the total area of cross-sectional microstructure (0.034mm²×30 viewing fields).

[2. Method for Manufacturing Precipitation Hardened Fe—Ni Alloy]

The method for manufacturing the precipitation hardened Fe—Ni alloyaccording to the invention comprises a melting and casting process, ahot working process, a solution heat treatment process, and an agingtreatment process.

[2.1. Melting and Casting Process]

The melting and casting process is a process of dissolving a rawmaterial blended in a predetermined composition and performing casting.The dissolving method and the casting method are not particularlylimited and various methods can be used according to the purpose.

[2.2. Hot Working Process]

The hot working process is a process of hot-working an ingot obtained inthe melting and casting process. The hot working is performed fordestroying cast structure and casting defect. Hot working conditions arenot particularly limited and most suitable conditions can be selectedaccording to the purpose.

[2.3. Solution Heat Treatment Process]

The solution heat treatment process is a process of heating a hot-workedmaterial at a predetermined temperature.

The solution heat treatment is performed mainly for dissolving aprecipitate dispersed in the steel. When heat treatment temperature istoo low, solution of the precipitate becomes insufficient. The heattreatment temperature is preferably 900° C. or higher.

On the other hand, when the heat treatment temperature is too high, thecrystal grains are coarsened. The heat treatment temperature ispreferably 1,200° C. or lower.

Heat treatment time may be suitably a time sufficient for dissolving theprecipitate. Most suitable heat treatment time varies depending on theheat treatment temperature but is usually from about 30 minutes to about2 hours. After the heat treatment, the material is quenched.

[2.4. Aging Treatment Process]

The aging treatment process is a process of subjecting the materialafter the solution heat treatment to an aging treatment at apredetermined temperature.

In both cases where aging treatment temperature is too high and too low,an objective precipitate is not precipitated and aging hardening cannotbe achieved. The aging treatment temperature is preferably from 600° C.to 750° C.

Aging treatment time may be suitably a time sufficient for precipitatinga sufficient amount of the precipitate. Most suitable aging treatmenttime varies depending on the aging treatment temperature but is usuallyfrom about 8 hours to about 24 hours.

[3. Action]

When a predetermined amount of Nb is added to the precipitation hardenedFe—Ni alloy, the γ′ phase (Ni₃(Al, Ti, Nb)) and the γ″ phase (Ni₃Nb)containing Nb as a constituent element are precipitated by the solutionheat treatment and the aging treatment.

At this time, when the Nb content is optimized so as to satisfy theformula (2), the precipitation amount of the γ″ phase is increased.Therefore, as compared with conventional alloys, high strength can beobtained.

On the other hand, as the addition amount of Nb increases, the Lavesphase (Fe₂Nb) tends to remain after the solution heat treatment. When alarge amount of the Laves phase remains, the Nb amount in the matrixnecessary for precipitation hardening decreases. As a result, necessaryhardness is not obtained even when the aging treatment is performed.

In contrast, when the Ni content is optimized so as to satisfy theformula (1), the remaining of the Laves phase after the solution heattreatment can be suppressed.

Furthermore, when predetermined amounts of Cr and Mo are added to theprecipitation hardened Fe—Ni alloy or predetermined amounts of Cr, Moand Cu are added to the precipitation hardened Fe—Ni alloy, highcorrosion resistance is obtained with maintaining high strength.Particularly, when the contents of Cr, Mo, and Cu are optimized so as tosatisfy the formula (3), high corrosion resistance is obtained.

EXAMPLES Examples 1 to 37, Comparative Examples 1 to 5 [1. Preparationof Samples]

After each steel containing various components shown in Tables 1 and 2was ingoted, each steel was cooled to prepare an ingot. After hotworking, the ingot was thermally refined by a solution heat treatmentand an aging treatment.

Solution heat treatment temperature was set to 900 to 1,200° C. Also,the aging treatment temperature was set to 600 to 750° C.

TABLE 1 Component balance Component composition (% by mass) FormulaFormula Formula C Si Mn Ni Cr Mo Al Ti Nb B Cu V Others (1′) (2′) (3′)Example 1 0.06 0.24 0.1 39.2 15.7 0.01 0.2 1.9 3.1 — — 3.6 0.68 −3.27Example 2 0.05 0.41 0.5 36.9 15.4 0.03 0.2 2.3 2.7 0.0051 0.07 — 3.70.28 −3.16 Example 3 0.06 0.46 0.3 39.8 15.8 0.13 0.5 2.1 3.2 0.00990.06 — 3.6 0.43 −2.51 Example 4 0.04 0.35 0.3 39.8 15.4 1.54 0.3 2.4 2.70.0041 0.07 — 6.6 0.20 1.37 Example 5 0.04 0.35 0.3 39.8 15.4 0.52 0.32.4 2.7 0.0025 0.07 — 6.6 0.20 −1.69 Example 6 0.05 0.39 0.6 36.8 15.02.40 0.2 2.2 2.7 0.0046 0.06 — 3.6 0.33 3.50 Example 7 0.04 0.35 0.341.0 15.4 1.54 0.3 2.2 2.7 0.0063 0.07 — 7.8 0.28 1.37 Example 8 0.040.35 0.3 38.9 15.4 0.53 0.3 2.2 2.7 0.0006 0.52 — 5.7 0.28 0.59 Example9 0.04 0.35 0.3 38.9 15.4 0.52 0.3 2.2 2.7 0.0051 0.07 0.5 5.7 0.28−1.69 Example 10 0.04 0.35 0.3 38.9 17.8 0.52 0.3 2.2 2.7 0.0099 0.07 —5.7 0.28 0.71 Example 11 0.04 0.35 0.3 38.9 19.5 0.52 0.3 2.2 2.7 0.00410.07 — 5.7 0.28 2.41 Example 12 0.04 0.35 0.3 36.4 15.4 0.52 0.3 2.2 2.70.0025 0.07 — 3.2 0.28 −1.69 Example 13 0.04 0.35 0.3 38.9 15.4 0.52 0.32.2 2.2 0.0046 0.07 — 8.7 0.08 −1.69 Example 14 0.04 0.35 0.3 38.9 15.40.52 0.3 2.2 3.4 0.0063 0.07 — 1.5 0.56 −1.69 Example 15 0.04 0.35 0.338.9 15.4 0.52 0.9 2.2 2.7 0.0006 0.07 — 5.7 0.07 −1.69 Example 16 0.040.35 0.3 38.9 15.4 0.52 0.3 1.3 2.7 0.0051 0.07 — 5.7 0.89 −1.69 Example17 0.04 0.35 0.3 38.9 15.4 0.52 0.3 2.4 2.7 0.0099 0.07 — 5.7 0.20 −1.69Example 18 0.04 0.82 0.3 38.9 15.4 0.52 0.3 2.2 2.7 0.0041 0.07 — Zr:0.05 5.7 0.28 −1.69 Example 19 0.04 0.35 0.3 38.9 15.4 0.52 0.3 2.2 2.70.0025 0.07 — Ta: 0.15 5.7 0.28 −1.69 Example 20 0.04 0.35 0.3 38.9 15.40.52 0.3 2.2 2.7 0.0046 0.07 — W: 0.32 5.7 0.28 −1.69 Example 21 0.040.35 0.3 38.9 15.4 0.52 0.3 2.2 2.7 0.0063 0.07 — Hf: 0.21 5.7 0.28−1.69 * Formula (1′) = Ni − (6 × Nb + 17), Formula (2′) = Nb/(Ti + Al) −0.8, Formula (3′) = Cr + 3Mo + 5Cu − 19

TABLE 2 Component balance Component composition (% by mass) FormulaFormula Formula C Si Mn Ni Cr Mo Al Ti Nb B Cu V Others (1′) (2′) (3′)Example 22 0.04 0.35 0.3 38.9 15.4 0.52 0.3 2.2 2.7 0.0006 0.07 — Mg:0.02 5.7 0.28 −1.69 Example 23 0.04 0.35 0.3 38.9 15.4 0.52 0.3 2.2 2.70.0046 0.07 — REM: 0.03 5.7 0.28 −1.69 Example 24 0.04 0.35 0.3 38.915.4 0.52 0.3 2.2 2.7 0.0063 0.07 — Ca: 0.004 5.7 0.28 −1.69 Example 250.04 0.35 0.3 38.9 15.4 1.62 0.3 2.2 2.7 0.0006 0.61 — 5.7 0.28 4.31Example 26 0.04 0.35 0.3 38.0 15.7 0.01 0.2 2.2 2.7 0.0011 — 4.8 0.33−3.27 Example 27 0.04 0.35 0.3 38.0 15.4 0.03 0.3 2.4 2.5 0.0023 — 6.00.13 −3.51 Example 28 0.04 0.35 0.6 38.0 15.8 0.13 0.2 2.2 2.7 0.00050.36 — 4.8 0.33 −1.01 Example 29 0.04 0.35 0.5 38.0 15.4 1.54 0.3 2.22.5 0.0042 0.09 — 6.0 0.20 1.47 Example 30 0.04 0.24 0.4 38.0 15.4 0.520.2 2.2 2.7 0.0051 — 4.8 0.33 −2.04 Example 31 0.04 0.24 0.5 38.0 17.80.53 0.3 2.2 2.5 0.0048 — 6.0 0.20 0.39 Example 32 0.05 0.24 0.6 38.019.5 0.52 0.2 1.5 2.7 0.0045 0.43 — 4.8 0.79 4.21 Example 33 0.05 0.460.3 38.0 15.4 0.52 0.3 2.2 2.5 0.0029 0.05 — 6.0 0.20 −1.79 Example 340.05 0.46 0.3 38.0 15.4 0.51 0.2 2.2 2.7 0.0055 0.3 4.8 0.33 −2.07Example 35 0.05 0.46 0.3 38.0 15.6 0.51 0.3 2.2 2.5 0.0061 0.4 6.0 0.20−1.87 Example 36 0.05 0.46 0.3 38.0 15.8 0.52 0.2 2.2 2.7 0.0053 0.570.4 4.8 0.33 1.21 Example 37 0.05 0.46 0.3 38.0 15.9 0.52 0.3 2.2 2.50.0048 0.5 6.0 0.20 −1.54 Comparative 0.03 0.31 0.2 25.1 15.0 1.1 0.22.2 — — 0.3 Example 1 Comparative 0.04 0.35 0.3 33.8 15.4 0.52 0.3 2.22.7 0.0041 0.07 — 0.6 0.28 −1.69 Example 2 Comparative 0.04 0.35 0.336.3 15.4 0.52 0.5 2.2 2.0 0.0041 0.07 — 7.3 −0.06 −1.69 Example 3Comparative 0.04 0.35 0.3 36.3 15.4 0.52 0.3 2.2 3.4 0.0041 0.07 — −1.10.56 −1.69 Example 4 Comparative 0.03 0.35 0.3 38.9 15.4 0.53 0.3 2.22.7 0.0045 0.07 — 5.7 0.28 −1.66 Example 5 * Formula (1′) = Ni − (6 ×Nb + 17), Formula (27) = Nb/(Ti + Al) − 0.8, Formula (3′) = Cr + 3Mo +5Cu − 19

[2. Test Methods] [2.1. Tensile Test]

From each material after the aging treatment, a JIS No. 4 test piece wascut out. A tensile test was performed at room temperature (20° C.) toevaluate tensile strength and 0.2% offset yield strength.

[2.2. Corrosion Resistance Test]

Evaluation of the corrosion resistance was performed on a corrosion rateat the time of immersion for 6 h in 10% hydrochloric acid at 80° C. Thecase where the corrosion rate was 100 g/m²/h or less was designated as“A”, the case where the rate was more than 100 g/m²/h and 200 g/m²/h orless was designated as “B”, and the case where the rate was more than200 g/m²/h was designated as “C”.

[2.3. Carbide Area Percentage]

For quantitative analysis of a carbide, area percentage was measured at30 visual fields on a microstructure photograph with a magnification of400 times (1 visual field: 0.034 mm²) using an image-analyzing software.

[3. Results]

Table 3 shows results. From Table 3, the following are realized.

-   (1) In Comparative Example 1 (corresponding to A286 alloy), the    tensile strength and the 0.2% offset yield strength are low. This is    because Nb is not added and the γ″ phase is not precipitated. Also,    in Comparative Example 1, the corrosion resistance is low. This is    because the Ni amount is small.-   (2) In Comparative Example 2, the 0.2% offset yield strength is    slightly low. This is because the Ni content is small and hence a    sufficient amount of the γ″ phase is not obtained. Also, in    Comparative Example 2, the corrosion resistance is low. This is    because the Ni amount is small.-   (3) In Comparative Example 3, the 0.2% offset yield strength is    slightly low. This is because a sufficient amount of the γ″ phase is    not obtained owing to a low value of Nb/(Ti+Al)−0.8.-   (4) In Comparative Example 4, the 0.2% offset yield strength is    slightly low. This is because a coarse Laves phase remains owing to    a low value of Ni−(6×Nb+17) and, as a result, the Nb amount in the    matrix is decreased and hence the precipitation amounts of the γ′    phase and the γ″ phase at the time of the aging treatment are    decreased.-   (5) In Comparative Example 5, the 0.2% offset yield strength is    slightly low. This is because the crystal grains are coarsened owing    to the small carbide area percentage, i.e., the small amount of the    carbide which suppresses crystal grain coarsening at the time of the    solution heat treatment.-   (6) In all of Examples 1 to 37, the 0.2% offset yield strength is    more than 850 MPa and good corrosion resistance is exhibited.-   (7) Among Examples, the materials satisfying the formula (3)    particularly exhibit high corrosion resistance.

TABLE 3 Tensile 0.2% offset Carbide area strength yield strengthCorrosion percentage (MPa) (MPa) resistance (%) Example 1 1168 972 B0.73 Example 2 1160 929 B 0.68 Example 3 1191 990 B 0.74 Example 4 1153961 A 0.51 Example 5 1196 946 B 0.53 Example 6 1137 894 A 0.59 Example 71163 972 A 0.48 Example 8 1160 969 A 0.49 Example 9 1193 945 B 0.54Example 10 1162 931 A 0.50 Example 11 1142 924 A 0.55 Example 12 1132866 B 0.52 Example 13 1175 888 B 0.47 Example 14 1227 997 B 0.61 Example15 1197 948 B 0.45 Example 16 1173 857 B 0.41 Example 17 1206 937 B 0.60Example 18 1198 948 B 0.52 Example 19 1187 936 B 0.54 Example 20 1203952 B 0.53 Example 21 1193 942 B 0.58 Example 22 1186 941 B 0.51 Example23 1188 937 B 0.54 Example 24 1192 945 B 0.49 Example 25 1137 894 A 0.55Example 26 1152 963 B 0.53 Example 27 1163 984 B 0.58 Example 28 1155966 A 0.42 Example 29 1148 957 B 0.47 Example 30 1166 987 B 0.54 Example31 1152 964 A 0.61 Example 32 1141 887 A 0.68 Example 33 1159 989 B 0.72Example 34 1164 977 B 0.71 Example 35 1161 983 B 0.66 Example 36 1142850 A 0.65 Example 37 1172 887 B 0.69 Comparative 1052 651 C 0.35Example 1 Comparative 1166 850 C 0.53 Example 2 Comparative 1004 843 C0.52 Example 3 Comparative 1018 827 C 0.55 Example 4 Comparative 1115846 B 0.38 Example 5

FIG. 1 shows optical microscopic photographs of materials after solutionheat treatment obtained in Example 5 and Comparative Example 4. FromFIG. 1, it is realized that the Laves phase is observed besides thecarbide in Comparative Example 4 but the Laves phase is not observed inExample 5.

While the mode for carrying out the present invention has been describedin detail above, the present invention is not limited to theseembodiments, and various changes and modifications can be made thereinwithout departing from the purport of the present invention.

This application is based on Japanese patent application No. 2013-106957filed May 21, 2013 and Japanese patent application No. 2014-039222 filedFeb. 28, 2014, the entire contents thereof being hereby incorporated byreference.

INDUSTRIAL APPLICABILITY

The precipitation hardened Fe—Ni alloy according to the invention can beused as members for excavation, automobile engine parts, thermal powergeneration plant members, and the like.

What is claimed is:
 1. A precipitation hardened Fe—Ni alloy having thefollowing constitutions: (1) the precipitation hardened Fe—Ni alloycomprising: from 0.01 to 0.08% by mass of C, from 0.02 to 1.0% by massof Si, not more than 1.0% by mass of Mn, from 36.0 to 41.0% by mass ofNi, 14.0 or more and less than 20.0% by mass of Cr, from 0.01 to 3.0% bymass of Mo, from 0.1 to 1.0% by mass of Al, from 1.0 to 2.5% by mass ofTi, and from 2.0 to 3.5% by mass of Nb, with the balance being Fe andunavoidable impurities; (2) the precipitation hardened Fe—Ni alloysatisfying the following formulae (1) and (2):Ni≧6×Nb+17   (1)Nb/(Ti+Al)≧0.8   (2).
 2. The precipitation hardened Fe—Ni alloyaccording to claim 1, further comprising from 0.0005 to 0.01% by mass ofB.
 3. The precipitation hardened Fe—Ni alloy according to claim 1,wherein an area percentage of a carbide after a solution heat treatmentis 0.4% or more.
 4. The precipitation hardened Fe—Ni alloy according toclaim 2, wherein an area percentage of a carbide after a solution heattreatment is 0.4% or more.
 5. The precipitation hardened Fe—Ni alloyaccording to claim 1, further comprising from 0.05 to 1.0% by mass ofCu.
 6. The precipitation hardened Fe—Ni alloy according to claim 1,further comprising from 0.05 to 1.0% by mass of V.
 7. The precipitationhardened Fe—Ni alloy according to claim 1, further comprising at leastone element selected from the group consisting of Zr, Ta, W, Hf, Mg, andREM, provided that a total content thereof is from 0.001 to 0.50% bymass.
 8. The precipitation hardened Fe—Ni alloy according to claim 1,further comprising from 0.0005 to 0.01% by mass of Ca.
 9. Theprecipitation hardened Fe—Ni alloy according to claim 1, wherein 0.2%offset yield strength at room temperature is 900 MPa or more.
 10. Theprecipitation hardened Fe—Ni alloy according to claim 1, furthersatisfying the following formula (3):Cr+3Mo+5Cu≧19   (3).